Know If the Valve Is Really Stuck
Check the O/B setting, coil voltage and pressure response before you condemn a reversing valve.
System Diagnostics
Solve Defrost and Mode-Change Faults Without Unnecessary Board Swaps
Heat pump problems that look random usually aren't. ACLogics walks you through the checks that show whether it's the thermostat, the wiring, the valve or a sensor.
FAULT SYMPTOM
The air conditioner blows cold air during the winter
CHECK 01
Initiate a forced defrost cycle on the board. Does it initiate?
Yes, defrost starts
DIAGNOSIS
Weak outdoor fan capacitor or worn fan motor causing premature cycle termination.
No, nothing happens
CHECK 02
Measure defrost sensor resistance in ice bath.
Out of range (Infinite/Short)Correct ohms (e.g., 10k/30k)
Complete diagnostic tree — every branch, every fault path
Try ACLogics Free — iPhone & AndroidWhat You'll Stop Guessing About
Know what failed before you order the part.
Fix Frost Problems Without Swapping the Board
Check sensor readings, board inputs and coil condition, so you replace a board only when it's the board.
Stop Backup Heat Running When It Shouldn't
Check staging, lockout settings and thermostat setup when backup heat short cycles or won't help.
How a Call Goes
01
Tell It What's Happening
Enter outdoor conditions, the mode called for and what the thermostat and outdoor board are doing.
02
Check the Likely Branch First
Run mode-specific checks to rule out thermostat setup, wiring, valve or sensor faults.
03
Prove It Works Both Ways
Confirm it heats and cools correctly and keep your readings in the service record.
Frequently Asked Questions
Unit is stuck in cooling while thermostat calls for heat. Is the reversing valve failed?
Not necessarily. Confirm thermostat O/B configuration and control voltage first. Misconfigured logic often mimics valve failure.
Outdoor coil frosts heavily but never enters defrost. Where should diagnosis start?
Start with coil sensor resistance and board input status, then verify timer logic and relay output. Sensor drift is a common hidden cause.
Auxiliary heat runs too often even in mild weather. Is the heat pump undercharged?
Charge can contribute, but staging and lockout settings are usually first. Verify thermostat balance-point logic and sensor calibration.
Suction pressure looks weak only in heating mode, not cooling mode. What does that indicate?
Focus on outdoor airflow, defrost residue, and heating-mode metering behavior. A heating-only airflow restriction is a common root cause.
Know What to Check Next on Your Next Call
Get the checks in order, confirm the cause, and fix it right the first time.